The present invention relates to a trussed structure, which may be preferably utilized for a long span roof framing of a building.
For long span roof framings, there are conventionally various structures used such as a trussed structure, shell structure, suspended structure, and pneumatic structure.
As prior art for the present invention, Japanese Patent Application No. 60-175997 (Publication No. 6237449), U.S. Pat. No. 4,697,397, and European Patent No. 0211671 are quoted. (The U.S. and European Patents are based on the Japanese Application.) The trussed girder in the patents enables to reduce its own weight and to cause relatively small deflection, so that cost performance is enhanced.
However, the trussed girder presents the following problems:
(1) Pre-tensioned steel cables are disposed along chord members. Therefore, it is very difficult, due to geometrical considerations, to fix the steel cables to desirable positions so that it will not cause a twisting force along the central axis of mass of the chord member in a case where the central axis of the chord member is inside the chord member.
(2) In order to fix the pre-tensioned steel cables to ends of the chord members, the cross-section of the ends are necessarily large.
(3) The out-of-plane strength is not sufficiently high in the trussed girder. If the span is larger than 50-60 m, it is difficult to first construct the whole structure on the ground and then move it into a subject position by a crane because of low buckling resistance.
(4) A continuous bending moment does not exist in supporting columns in the trussed girder. However, a large continuous bending moment exists on each of the bases of the columns when the trussed girder receives a wind load or a superimposed load. The base construction is therefore uneconomical.
Accordingly, a trussed girder, described in Japanese Patent Application No. 63-129221 (Publication No. 1-299943), U.S. patent application Ser. No. 356,442, U.K. Patent Application No. 8911966.3, is proposed. (The U.S. and U.K. Applications are based on the Japanese Application.)
The structure disclosed in the above patent applications comprises a trussed girder interconnecting between columns, and a pre-tensioned steel cable disposed within and along the trussed girder. The trussed girder includes a pair of upper chord members and a lower chord member, therefore said girder is of a reversed triangle cross section. The ends of the pre-tensioned steel cable are fixed to the columns. The medium portion of the pre-tensioned steel cable is fixed to the lower chord member which is below the ends of the pre-tensioned steel cable. By virtue of the pre-tensioned steel cable, the trussed girder receives an upward thrust force. When introducing the prestress to the pre-tensioned steel cable, first the upper chord members are disposed on but not fixed to the columns. Then, the upper chord members are fixed to the columns to produce a rigid rahmen structure. The columns and the trussed girder cooperate to resist a superimposed load which may occur for a short time (for example, earthquakes, snows, hurricanes, or the like).
With such a three dimensional structure, a sufficient out-of-plane strength is achieved so that the lower chord member is prevented from buckling. Consequently, a large span structure can be produced. In addition, the pre-tensioned steel cable can be disposed in an optional manner so that the cross sectional area of the trussed girder is decreased.
However, the structure still has problems as follows: Since the trussed girder is of a reversed triangle cross section, the arrangement of the lower chord member and lattice members is complicated, so that the construction process is troublesome. Moreover, the chord members, when spanning between the columns, may become unstable.